1. Field of the Invention
The present invention relates to a catalytic hydrocracking process, and particularly to a hydrocracking process for the production of high octane gasoline and improved mid-distillates from substantially dealkylated, highly refractory, aromatic and low quality mid-distillate feedstocks. The present invention is also related to recycling upgraded fractions from the hydrocracking step to a fluid catalytic cracking unit.
2. Discussion of Prior Art
The economic viability of petroleum refineries increasingly relies on the ability to convert the greatest proportion of a given barrel of crude to premium fuels, such as gasoline, diesel and jet fuel. Catalytic cracking processes, exemplified by the fluid catalytic cracking (FCC) process and thermofor catalytic cracking (TCC) process together, account for a substantial fraction of heavy liquids conversion in modern refineries. Both are thermally severe processes, wherein the intrinsic thermal reactivity of high boiling virgin streams is of consequence. In particular, high molecular weight liquids disproportionate into relatively hydrogen rich light liquids and aromatic, hydrogen deficient heavier distillates.
Catalytic cracking in the absence of hydrogen is not an effective route to desulfurized liquids, nor is the nitrogen content of these feedstocks selectively rejected to coke. Both sulfur and nitrogen can thus concentrate appreciably in the heavier distillates derived from such primary conversion processes. Thus, these processes produce significant quantities of highly aromatic hydrogen dificient middle and heavy distillates that have high sulfur and nitrogen levels. Recycling these liquids to the catalytic cracker is often not an attractive option, because they are refractory and difficult to convert and often will impair conversion of the less refractory, nonrecycled feedstock to the catalytic cracker.
Examples of poor quality catalytic cracker refinery streams can include: light and heavy cycle oils and clarified slurry oil or main column bottoms. The following table lists two examples of such poor quality streams.
______________________________________ Aromatics % S ppm N % H ______________________________________ Light Cycle Oil 80 3.1 650 9.1 Main Column Bottoms G 80 4.6 1500 6.8 ______________________________________ G = Greater Than
Today's changing market requirements make these refractory streams particularly difficult to convert to commercially valuable products. Formerly, the light and heavy cycle oils from the catalytic cracking operation could be upgraded and sold as light or heavy fuel oil, such as No. 2 fuel oil or No. 6 fuel oil. Upgrading these oils conventionally utilizes a relatively low severity operation in a low pressure catalytic desulfurization unit, where the cycle stock would be admixed with virgin mid-distillates from the same crude blend fed to the catalytic cracker. Further discussion of this conventional technology is provided in the Oil and Gas Journal, May 31, 1982, pp. 87-94.
Currently, the refiner is finding a diminished demand for petroleum derived fuel oil. At the same time, the impact of changes in supply and demand for petroleum has resulted in a lowering of the quality of the crudes available to the refiner; this has resulted in the formation of an even greater quantity of refractory hard-to-upgrade cycle stocks than before. As a result, the refiner is left in the position of producing increased amounts of poor quality cycle streams from the catalytic cracker while having a diminishing market in which to dispose of these streams.
An alternative market for mid-distillate streams is automotive diesel fuel. However, diesel fuel has to meet a cetane number specification of about 45 in order to operate properly in typical automotive diesel engines. As is well known in the art, cetane number correlates closely with aromatics content. Refractory cycle oils can have aromatic contents as high as 80% or even higher, resulting in cetane numbers as low as 4 or 5. In order to raise the cetane number of the cycle stock to a satisfactory level by the conventional technology disclosed earlier, substantial and uneconomic quantities of hydrogen and high pressure processing would be required.
One relatively obvious and commonly practiced alternative route to convert or upgrade these streams is to severely hydrotreat prior to recycle to the catalytic cracker, or alternatively severely hydrotreat and feed to a high pressure hydrocracker. In such cases, the object of hydrotreating is to reduce heteroatoms, e.g., sulfur and nitrogen, to very low levels while saturating polyaromatics. Although this does enhance the convertibility of aromatic streams considerably, the economic penalties derived from high hydrogen consumptions and high pressure processing are severe. In addition, in those instances where the production of gasoline is desired, the naphtha may require reforming to recover its aromatic character and meet octane specifications.
There is a substantial amount of prior art in the field of hydrocracking heavy oils over a noble metal containing zeolite catalyst. For example, U.S. Pat. No. 3,132,090 discloses the use of a two-stage hydrocracking scheme to produce high octane gasoline. However, the octane of the gasoline using a virgin distillate as charge was reported as 68 (RON+0). In the same disclosure, an octane of 80 (RON+3) was disclosed for a chargestock of coker distillate and thermally cracked gas oils. All of the "high octane" gasoline cited in the '090 patent contain 3 ml of tetraethyl lead (TEL) and are in the range of 70-88 (RON+3). TEL can add 4-6 octane numbers to gasoline; therefore, on a clear basis, these octanes are in the range of 65-83 (RON+0).
U.S. Pats. Nos. 3,554,899; 3,781,199; 3,836,454; 3,897,327; 3,929,672; and 4,097,365 disclose catalysts and processes involving the use of palladium on various forms of zeolite Y catalysts. However, these disclosures fail to see the unobvious feedstock requirement of being substantially dealkylated in order to obtain high octane gasoline.
U.S. Pats. Nos. 3,867,277 and 3,923,640 both disclose low pressure hydrocracking processes. However, the object of these disclosures is not to produce high octane gasoline, because they also fail to note the requirement to use substantially dealkylated feedstock in order to obtain high octane gasoline.
Although it is acknowledged that the above-referenced patents disclose processes which produce desirable product fuels, substantial improvements can be made in product quality in terms of higher octane number and increased cetane. It has unexpectedly been found that substantial improvements in terms of octane number and distillate quality can be made by utilizing a specific feedstock under specified conditions.